asio::async_read

The async_read function is a composed asynchronous operation that reads a certain amount of data from a stream before completion. More...

Functions
template<typename AsyncReadStream , typename DynamicBuffer_v1 , ASIO_COMPLETION_TOKEN_FOR(void(asio::error_code, std::size_t)) ReadHandler>
	asio::ASIO_INITFN_AUTO_RESULT_TYPE (ReadHandler, void(asio::error_code, std::size_t)) async_read(AsyncReadStream &s
	Start an asynchronous operation to read a certain amount of data from a stream. More...
template<typename AsyncReadStream , typename MutableBufferSequence , typename CompletionCondition , ASIO_COMPLETION_TOKEN_FOR(void(asio::error_code, std::size_t)) ReadHandler ASIO_DEFAULT_COMPLETION_TOKEN_TYPE( typename AsyncReadStream::executor_type) >
const MutableBufferSequence ASIO_MOVE_ARG(ReadHandler) handler ASIO_DEFAULT_COMPLETION_TOKEN(typename AsyncReadStream	asio::ASIO_INITFN_AUTO_RESULT_TYPE (ReadHandler, void(asio::error_code, std::size_t)) async_read(AsyncReadStream &s
	Start an asynchronous operation to read a certain amount of data from a stream. More...
	asio::ASIO_MOVE_ARG (DynamicBuffer_v1) buffers

Variables
const MutableBufferSequence &	asio::buffers
const MutableBufferSequence CompletionCondition	asio::completion_condition
enable_if< is_dynamic_buffer_v1< typename decay< DynamicBuffer_v1 >::type >::value &&!is_dynamic_buffer_v2< typename decay< DynamicBuffer_v1 >::type >::value >::type CompletionCondition enable_if< is_dynamic_buffer_v1< typename decay< DynamicBuffer_v1 >::type >::value &&!is_dynamic_buffer_v2< typename decay< DynamicBuffer_v1 >::type >::value >::type basic_streambuf< Allocator > &	asio::b

Detailed Description

The async_read function is a composed asynchronous operation that reads a certain amount of data from a stream before completion.

Function Documentation

ASIO_INITFN_AUTO_RESULT_TYPE() [1/2]

template<typename AsyncReadStream , typename DynamicBuffer_v1 , ASIO_COMPLETION_TOKEN_FOR(void(asio::error_code, std::size_t)) ReadHandler>

asio::ASIO_INITFN_AUTO_RESULT_TYPE ( ReadHandler  , void(asio::error_code, std::size_t)    ) &

inline

Start an asynchronous operation to read a certain amount of data from a stream.

Start an asynchronous operation to read data into a dynamic buffer sequence until it contains a specified delimiter.

Start an asynchronous operation to read a certain amount of data at the specified offset.

This function is used to asynchronously read a certain number of bytes of data from a stream. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

• The supplied buffers are full. That is, the bytes transferred is equal to the sum of the buffer sizes.

• An error occurred.

This operation is implemented in terms of zero or more calls to the stream's async_read_some function, and is known as a composed operation. The program must ensure that the stream performs no other read operations (such as async_read, the stream's async_read_some function, or any other composed operations that perform reads) until this operation completes.

Parameters

s	The stream from which the data is to be read. The type must support the AsyncReadStream concept.
buffers	One or more buffers into which the data will be read. The sum of the buffer sizes indicates the maximum number of bytes to read from the stream. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the handler is called.
handler	The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be: void handler( const asio::error_code& error, // Result of operation. std::size_t bytes_transferred // Number of bytes copied into the // buffers. If an error occurred, // this will be the number of // bytes successfully transferred // prior to the error. ); Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using asio::post().

Example

To read into a single data buffer use the asio::buffer function as follows:

asio::async_read(s, asio::buffer(data, size), handler);

See the asio::buffer documentation for information on reading into multiple buffers in one go, and how to use it with arrays, boost::array or std::vector.

Note

This overload is equivalent to calling:

asio::async_read(
   s, buffers,
   asio::transfer_all(),
   handler); 

This function is used to asynchronously read a certain number of bytes of data from a random access device at the specified offset. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

• The supplied buffers are full. That is, the bytes transferred is equal to the sum of the buffer sizes.

• An error occurred.

This operation is implemented in terms of zero or more calls to the device's async_read_some_at function.

Parameters

d	The device from which the data is to be read. The type must support the AsyncRandomAccessReadDevice concept.
offset	The offset at which the data will be read.
buffers	One or more buffers into which the data will be read. The sum of the buffer sizes indicates the maximum number of bytes to read from the device. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the handler is called.
handler	The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be: void handler( // Result of operation. const asio::error_code& error, // Number of bytes copied into the buffers. If an error // occurred, this will be the number of bytes successfully // transferred prior to the error. std::size_t bytes_transferred ); Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using asio::post().

Example

To read into a single data buffer use the asio::buffer function as follows:

asio::async_read_at(d, 42, asio::buffer(data, size), handler);

See the asio::buffer documentation for information on reading into multiple buffers in one go, and how to use it with arrays, boost::array or std::vector.

Note

This overload is equivalent to calling:

asio::async_read_at(
   d, 42, buffers,
   asio::transfer_all(),
   handler); 

This function is used to asynchronously read data into the specified dynamic buffer sequence until the dynamic buffer sequence's get area contains the specified delimiter. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

• The get area of the dynamic buffer sequence contains the specified delimiter.

• An error occurred.

This operation is implemented in terms of zero or more calls to the stream's async_read_some function, and is known as a composed operation. If the dynamic buffer sequence's get area already contains the delimiter, this asynchronous operation completes immediately. The program must ensure that the stream performs no other read operations (such as async_read, async_read_until, the stream's async_read_some function, or any other composed operations that perform reads) until this operation completes.

Parameters

s	The stream from which the data is to be read. The type must support the AsyncReadStream concept.
buffers	The dynamic buffer sequence into which the data will be read. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the handler is called.
delim	The delimiter character.
handler	The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be: void handler( // Result of operation. const asio::error_code& error, // The number of bytes in the dynamic buffer sequence's // get area up to and including the delimiter. // 0 if an error occurred. std::size_t bytes_transferred ); Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using asio::post().

Note

After a successful async_read_until operation, the dynamic buffer sequence may contain additional data beyond the delimiter. An application will typically leave that data in the dynamic buffer sequence for a subsequent async_read_until operation to examine.

Example

To asynchronously read data into a std::string until a newline is encountered:

 std::string data;
...
void handler(const asio::error_code& e, std::size_t size)
{
  if (!e)
  {
    std::string line = data.substr(0, n);
    data.erase(0, n);
    ...
  }
}
...
asio::async_read_until(s, data, '\n', handler); 

After the async_read_until operation completes successfully, the buffer data contains the delimiter:

{ 'a', 'b', ..., 'c', '\n', 'd', 'e', ... } 

The call to substr then extracts the data up to and including the delimiter, so that the string line contains:

{ 'a', 'b', ..., 'c', '\n' } 

After the call to erase, the remaining data is left in the buffer data as follows:

{ 'd', 'e', ... } 

This data may be the start of a new line, to be extracted by a subsequent async_read_until operation.

ASIO_INITFN_AUTO_RESULT_TYPE() [2/2]

template<typename AsyncReadStream , typename MutableBufferSequence , typename CompletionCondition , ASIO_COMPLETION_TOKEN_FOR(void(asio::error_code, std::size_t)) ReadHandler ASIO_DEFAULT_COMPLETION_TOKEN_TYPE( typename AsyncReadStream::executor_type) >

const MutableBufferSequence ASIO_MOVE_ARG (ReadHandler) handler ASIO_DEFAULT_COMPLETION_TOKEN( typename AsyncReadStream asio::ASIO_INITFN_AUTO_RESULT_TYPE	(	ReadHandler	,
		void(asio::error_code, std::size_t)
	)		&

Start an asynchronous operation to read a certain amount of data from a stream.

This function is used to asynchronously read a certain number of bytes of data from a stream. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

• The supplied buffers are full. That is, the bytes transferred is equal to the sum of the buffer sizes.

• The completion_condition function object returns 0.

Parameters

s	The stream from which the data is to be read. The type must support the AsyncReadStream concept.
buffers	One or more buffers into which the data will be read. The sum of the buffer sizes indicates the maximum number of bytes to read from the stream. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the handler is called.
completion_condition	The function object to be called to determine whether the read operation is complete. The signature of the function object must be: std::size_t completion_condition( // Result of latest async_read_some operation. const asio::error_code& error, // Number of bytes transferred so far. std::size_t bytes_transferred ); A return value of 0 indicates that the read operation is complete. A non-zero return value indicates the maximum number of bytes to be read on the next call to the stream's async_read_some function.
handler	The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be: void handler( const asio::error_code& error, // Result of operation. std::size_t bytes_transferred // Number of bytes copied into the // buffers. If an error occurred, // this will be the number of // bytes successfully transferred // prior to the error. ); Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using asio::post().

Example

To read into a single data buffer use the asio::buffer function as follows:

asio::async_read(s,
   asio::buffer(data, size),
   asio::transfer_at_least(32),
   handler); 

See the asio::buffer documentation for information on reading into multiple buffers in one go, and how to use it with arrays, boost::array or std::vector.